The distinct characteristic of rheological fluids is that, when exposed to the appropriate energy field, solid particles in the fluid align. When this alignment occurs, the ability of the fluid to flow, or shear, is substantially reduced. Two types of rheological fluids have been developed. Each is based on different applied energy fields. One fluid type is responsive to a voltage field, while the other is rheological to a magnetic field. Electro-rheological (ER) fluids, i.e., those that are responsive to a high-voltage, low-current electric field, have been extensively investigated. Some application restrictions are that the ER fluids require thousands of volts for operation, and yield low shear stresses. Safety and packaging are also design problems. On the other hand, electro-rheological magnetic (ERM) fluid, which is responsive to a magnetic field, operates on battery voltage and generates high fluid shear stress. The value of ERM fluid shear stresses developed are extremely high in comparison to ER fluid. ERM fluid has been used in automotive applications, such as shock absorbers, clutches, engine mounts and active bushings. ERM fluid typically includes a magnetizable particulate, fibrous carbon and a carrier vehicle. An example of a solid magnetizable particle is carbonyl iron. Other ferrometallic particulates and compounds may also be used. The carrying vehicle can be silicone oil. The fluid may also contain a surfactant to keep the solid particles in suspension. The application requirements for magnetic field-responsive fluids include very low shear resistance at zero field, high shear stresses at maximum applied field, very low hysteresis, chemical inertness, temperature stability and fast response time.
Conventional exercise machines typically use the force of gravity acting on a stack of weights to apply a resistance force to the movement of a body part to strengthen the muscles controlling such part. The user selects the number of weights desired for stressing the muscle involved in the movement of the body part in question. Alternatively, weights may be supported at variable distances along a force beam so that the resistance force applied to the user's body part is increased by the distance at which the weight is positioned from the pivot point of the force beam. Also, resistance to movement force is often varied during certain ranges of the exercise motion by using a cam to vary the effective weight of a weight stack or the length of a force movement arm of the device. The greatest deficiencies of this type of conventional exercise machine is that it is subject to the effects of gravity, friction and inertia. The combination of these three forces cause the actual user-experienced force to be less than predictable except within very limited performance parameters. Similar difficulties are encountered when the weights are replaced with elastic members or by fluid resistance or force-type components, such as air flow fans or hydraulic pistons or motors. None of the conventional exercise machines have the ability to substantially increase or decrease the selected applied force other than by a slight variation caused by the changing leverages resulting from changes in the structural configuration relating one part of the exercise machine to another. None of the conventional machines offers an accurate predictable force at varied exercise speeds and few have the ability to offer a controlled resistance in both directions of the exercise stroke, such as muscle extension followed by muscle retraction or contraction.
Several electronically controlled exercise machines are known where forces are created by an electromagnetic braking system, a hydraulic force system, a pneumatics force system or a direct current motor used as a dynamic brake. The electromagnetic braking systems target the weakest force generated by a muscle and consequently the results from the exercise machine are limited to the relatively low force resistance of the muscles. The air hydraulics or pneumatics system and the fluid hydraulics system of applying resistive forces in exercise machines have been used by several manufacturers. These machines allow resistance in both directions of the exercise stroke. These types of exercise machines use air or fluid pressure and mechanical linkages or leverage systems to provide the resistance forces against which exercise forces are applied by the user. Both systems are quite expensive to produce and their overall speed and force potential are not controllable to the extent desired. Further, these systems are often large and bulky and have a potential for fluid leaks, having bubbles form in the fluid channels, and require systematic maintenance to assure correct operation. A direct current motor has recently been used as a dynamic braking device in exercise machines. This method of producing a resistance force is rather basic and is not easily adaptable to even simple force curves. Further, exercise machines with dynamic braking devices having a problem with inertia and thereby may be less safe in operation. Inertia also reduces response time to electric commands from the control system and consequently reduces the performance of the mechanism.